Drive slip control systems are known from the existing art in many variations. For example, European Patent No. 0 1 63 941 (corresponding to U.S. Pat. No. 4,625,824) describes a drive slip control system in which, when incipient slippage of at least one driven wheel is detected, that wheel is braked and/or the drive unit of the vehicle is caused to reduce its torque. In the low wheel rotation speed rangee, i.e. in the initial vehicle movement range, only the wheel which first exhibits incipient slippage is braked, while the other driven wheel, even if incipient slippage is present, remains unaffected until a threshold value for the rotation speed of the drive unit has been exceeded.
Although the vehicle's initial movement behavior has been considerably improved by this procedure, it is still not satisfactory in certain applications. This is evident especially from the fact that, for example on a sloping road surface, the vehicle can slide laterally if the second, uncontrolled wheel becomes unstable.
It is an object of the present invention to improve a drive slip control system, especially in the initial vehicle movement range, in terms of the driving comfort and/or stability of the vehicle.
German Patent No. 43 44 634 (corresponding to U.S. Pat. No. 5,696,683) describes a drive slip controller in which the torque demand or torque reduction of the drive is ascertained from vehicle resistance torques. If an instability occurs, the drive torque of the vehicle's drive unit is reduced by the engine controller to that torque demand, in order to reestablish stability at the driven wheels.